1. Technical Field
The present invention refers, in general, to the technical field of security systems and, in particular, concerns a seismic sensor for a security system, as defined in the preamble of the first claim. The present invention also concerns a security system including such a sensor.
2. Description of the Related Art
As known, for a long time there has been a great need to protect the perimeter of buildings, with immediate indication, through suitable emission of an alarm, of a possible attempt to gain unwarranted access to a building being protected or of a possible attempt to escape from it.
Security systems, known as underground systems, which use various types of sensors, intended to be arranged in the ground or incorporated in the flooring and along the perimeter of a building to be protected, or along its potentially accessible areas are known and extensively used. In practice, the sensors of underground systems are seismic sensors generally sensitive to footsteps on the ground or on the flooring and deriving from a person approaching the perimeter or the protected area.
A particular security system of the type described above is disclosed by international patent application published as WO 96/10195, which in particular discloses a security system including a plurality of pressure sensors, or seismic sensors, equipped with a piezoelectric transducer.
In the seismic sensors disclosed by the aforementioned patent application, the seismic waves that propagate through the ground, caused by the passing of a person at the surface and near to the sensor, produce the movement of a suitable “receptor”, the pivot point of which is indirectly coupled through soft resin with a piezoelectric transducer housed in the base of the sensor.
The movement of the receptor with respect to the base of the sensor causes a weak mechanical stress at the piezoelectric membrane, which thus produces an electrical impulse that is then amplified and analyzed by suitable electronic circuits.
The piezoelectric transducer is characterized by a high conductivity of two of its conductive surfaces insulated from each other by a very fine insulating layer.
It has been observed that the correct operation of the piezoelectric transducer and its lifetime essentially depend upon two factors:                a stable working position, which does not involve the application of high mechanical stress to the transducer, so as to prevent it from breaking; and        perfect electrical insulation of the two conductive surfaces of the transducer, both between each other and against the surrounding environment (ground).        
The mechanical protection of the transducer is obtained by designing the base of the sensor like a rigid container that houses the transducer leaving just its upper free surface accessible. The coupling between transducer and receptor occurs indirectly, through a thick layer of polyurethane or epoxy resin arranged on the free surface of the transducer, so as to prevent it from breaking due to a possible direct contact with the pin of the receptor.
The electrical insulation is ensured by the same resin that effectively also carries out the function of sealing and electrically insulating the transducer from the surrounding environment from infiltrations of water/humidity.
With this type of solution the protection from humidity relies totally on the integrity of the layer of sealing resin that, in the long term, due to thermal shifts and perhaps due to contact with particular chemical substances that may be present in the ground, can deteriorate and lose its adhesion.
Moreover, the different expansion of the materials (resin and plastic of the casing) caused by the continuous thermal cycles (night/day, winter/summer), can cause the resin to detach from the casing and consequently water channels to be formed on the edges.
For greater protection from infiltrations a thick layer of additional sealing material, sufficiently soft to allow the receptor to move with respect to the base, has been foreseen along the edges defined between the receptor and the base of the sensor. Such a sealing material is the only barrier against the penetration of water or humidity inside the sensor.